Transparent conductive films are finding widespread use in such applications as electronic and optical devices that exploit the films' transparency, conductivity, and stability. Providing transparent conductive films as coatings on flexible transparent substrates broadens their range of use.
Some flexible substrates that may be in common use in other coatings applications have found limited use for transparent conductive film applications. Standard grades of polyethylene terephthalate (PET) films, for example, can exhibit excessive migration of oligomers to film surfaces, often leading to increased haze due to the powdered deposits that can develop during processing.
Flexible films based on polyethylene naphthalate (PEN) are reported to exhibit low levels of oligomer migration and haze development relative to standard grades of PET film. See, for example, U.S. Pat. No. 4,725,479 to Utsumi, which is hereby incorporated by reference in its entirety. However, PEN films have higher raw material costs and can be more costly to manufacture than standard grades of PET films; they are generally sold at a premium.
Specialty grades of PET films that have low oligomer content are known. See, for example, U.S. Pat. No. 5,498,454 to Kuze et al. and U.S. Pat. No. 6,020,056 to Walker et al., both of which are hereby incorporated by reference in their entirety. Resin oligomer content can be reduced after melt-phase polymerization, for example, by thermally processing the resin at temperatures between the resin's glass transition and melting temperatures, such as during solid-state polymerization processes, resin annealing processes, oligomer extraction processes, and the like. Further oligomer development after such processing can be limited by, for example, reducing film processing temperatures and dwell times. Such grades of PET film tend to be more costly to manufacture than standard grades of PET films and are generally sold a premium.